1. Field of the Invention
The present invention relates to a method for producing a resin-molded substrate and a method for producing a reversible(in other word, reusable) image display medium, in which image displaying and image erasing operations can be repeated.
2. Description of the Background Art
At present, image display is performed, e.g., in the following manners. A person uses a pencil, a pen, paints or the like, and manually writes or draws characters, pictures or the like on an image display medium such as paper sheet. Also, a computer, a word processor or the like is used to display text, graphics or the like on a display such as a CRT display, or output them on a medium such as a paper sheet via a printer for display.
A copying machine or the like may be used for producing duplication, on a medium of paper or the like, of the texts, pictures, graphics or the like, which are produced on the medium of paper or the like by a person or by a printer. A facsimile machine may be used for sending such contents (texts, pictures, graphics and others) prepared in the above manner for producing duplication on another medium of paper or the like.
The above image display, which is performed to display the texts, pictures or the like on the image display medium of paper or the like by a pencil, pen or the like, or by an image forming apparatus such as a printer, a copying machine or a facsimile machine operating in a electrophotographic method, an ink-jet method, a heat transfer method or the like, can achieve clear image display in a high resolution, and thus can achieve easy-on-the-eyes display.
However, it is impossible to repeat display and erasure of the images on the image display medium of paper or the like. In the case where the paper is used for writing characters or the like by a pencil, the characters can be erased by an eraser to a certain extent. However, it is difficult to erase completely the characters or the like written in an ordinary density, although it may be possible when written in a light density. The medium of paper or the like can not be reused except for the case of using the rear surface of the medium, which is not yet used for the image display.
Accordingly, the medium of paper or the like bearing images will be abandoned or burnt when it is not longer required. This results in consumption of a large mount of resources. The printer, copying machine or the like also consume consumable products or materials such as toner or ink. For obtaining the new display medium of paper or the like as well as toner, ink or the like, energies and resources are required for producing them. This is contrary to the current demand for reduction in environmental loads.
In contrast to the above, the image display by a display such as a CRT display can repeat the image display and the image erasure. However, the resolution, clarity and precision of images are restricted, as compared with the images displayed by the printer or the like on the paper medium or the like. Thus the image display by the display such as the CRT display is improper especially when used for displaying the text documents mainly composed of letters because of low resolution. If it is used for displaying sentences which continue in less than the frame-size volume, it will do. However, if the sentences continue in twice or more times the frame-size volume, they may be difficult to read and to understand. Due to the relatively low resolution and the light emission from the display, operations for a long time are likely to be hard to eyes.
Electrophoretic display (EPD) and Twist ball-type display (TBD) have been proposed as an image display method allowing repetition of the image display and image erasure. Further displaying method was recently proposed, which is disclosed in “Japan Hardcopy '99, the book of the thesis, pp. 249-252”.
In the electrophoretic display method, two substrates including at least one transparent substrate are opposed together with a spacer therebetween to form a closed space therebetween, and the space is filled with a display liquid formed of a dispersion medium and electrophoretic particles, which are dispersed in the dispersion medium and are different in color from the medium. The image display is performed by an application of an electrostatic field and in a color of the particles or a color of the dispersion medium.
The display liquid is usually formed of isoparaffin-contained dispersion medium, particles of titanium dioxide or the like, dyes applying contrast in color to the particles, and an additive such as a surface active agent, or a charge applying agent.
In the electrophoretic display, the display is performed by utilizing contrast between particles of a high refractive index (e.g., titanium dioxide particles) and colored insulating liquid, and therefore the particles can not hide the colored liquid to a high extent, resulting in a low contrast.
Furthermore, there is a limitation on the kind of dye which is dissolved in a high concentration in a nonpolar solvent of high resistance which allows the electrophoresis of particles. A dye showing a white color is not found. Nor known is a black dye having a high extinction coefficient. Therefore the background portion becomes colored so that it is difficult to achieve a good contrast by a white background. When white particles for formation of images are placed into a colored liquid, the colored liquid may be moved between the substrate and the layer of white particles which are moved to the image observation side substrate, or the colored liquid may come into between the white particles, thereby lowering the contrast. The electrophoretic particles can scarcely uniformly adhere to the image observation side substrate, and thus the resolution is low.
Further, settling and condensation of particles are liable to occur due to a very large difference in specific gravity between the particles and the dispersion medium in the display liquid. This is liable to lower the display contrast. Further, it is difficult to display the images with high stability for a long time, and remaining of last images is liable to occur. Further, the degree of charging of the particles in the liquid significantly changes with time, which also impairs the stability of the image display.
In the twist ball-type display method, images can be displayed in specified colors using an image display medium containing numerous microcapsules enclosing not only an insulating liquid but also fine spheric particle so processed that a half of their surface and the other surface portion show colors or an optical density which differs from each other. Images are displayed in predetermined colors by rotating the fine spheric particles in the microcapsules due to an electric field strength or magnetic strength.
However, according to the twist ball-type display, images are displayed using fine spherical particles in the insulating liquid within the microcapsules. This makes it difficult to attain good contrast. Further, the resolution is low since spaces are formed between the microcapsules. In the manufacture of microcapsules, difficulty is entailed in reducing the size of microcapsules to increase the resolution.
The “Japan Hardcopy '99, the book of the thesis, pp. 249-252” discloses an image displaying method wherein a closed space is formed by placing two substrates as opposed to each other and as spaced from each other, i.e. the two substrates being a laminate of electrodes and a charge transporting layer, the space being used to enclose the electrically conductive toner and insulating particles which are different in color from the toner, an electrostatic field being applied to inject charges into the electrically conductive toner so that the toner is moved by a Coulomb force applied thereto to display images.
However, the foregoing image displaying method utilizing the charge injection phenomenon poses problems. When the electrically conductive toner particles carrying the injected charges, are moved, insulating particles (e.g. white particles mixed together to form the color of background) interfere with the movement of the toner particles, making their movement so difficult that some of them may stop their movement. This results in failure to obtain satisfactory image density and good contrast and in reduction of image display rate. To overcome these problems, a high voltage drive is necessitated. The resolution is determined by the electrodes and is so limited. Furthermore, it is essential to use electrodes, charge-injection layer and electrically conductive toner, which results in limited manufacture.
A few of the present inventors have devised together with other inventors a reversible image display medium having a basic structure given below to overcome the foregoing problems. In contrast to such conventional image display mediums, the devised image display medium aims:    (1) to allow repeated image display and image erasure, and to reduce the consumption of image display mediums of paper or the like in the prior art and consumable materials such as developers and ink so that a current demand for reduction in environmental loads can be satisfied,    (2) to perform image display in good contrast and therefore in high quality,    (3) to perform image display in high resolution and therefore in high quality,    (4) to achieve stable image display for a long time,    (5) to suppress remaining of last images and therefore to exhibit good reversibility so that high quality image display can be achieved,    (6) to allow quick image display, and    (7) to reduce a drive voltage required for image display.
The reversible image display medium has the following basic structure. It comprises:
two substrates opposed to each other with a predetermined gap therebetween;
one or more developer-accommodating cells formed between the two substrates, each having a periphery surrounded by a partition wall; and
a dry developer contained in each of the cell(s), wherein
the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities.
This kind of image display medium may be hereinafter referred to as “developer-containing type reversible image display medium”.
The developer-containing type reversible image display medium is such that an electrostatic field corresponding to an image to be displayed is formed and applied to the frictionally charged developing particles accommodated in each cell of the image display medium. Thereby a Coulomb force can move the developing particles to display the image.
The electrostatic field corresponding to the image to be displayed can be formed, e.g., by applying a voltage corresponding to the image to be formed between electrodes arranged on the substrates of the medium or by forming an electrostatic latent image corresponding to the image to be formed on one of the substrates.
Such reversible image display medium comprises the two substrates opposed to each other with the predetermined gap therebetween; one or more developer-accommodating cells formed between the two substrates, each having the periphery surrounded by the partition wall; and the dry developer contained in each of the cell(s), wherein the dry developer contains at least two kinds of frictionally chargeable dry developing particles having different chargeable polarities and different optical reflection densities (in other words, exhibiting “different contrasts” or “different colors”).
Consequently, even after displaying the image, a different electrostatic field, an alternating electric field, an oscillating magnetic field (when magnetic developing particles are employed) or the like may be formed so that the image can be erased. Also, the image can be rewritten by forming a different electrostatic field. Accordingly, it is not necessary to abandon the image display medium, on which the image is already displayed. The developing particles are contained in the cell, and therefore external supply of the developer is not required. Owing to the features, it is possible to reduce remarkably the use of the image display medium such as paper sheets as well as consumable materials such as a developer in the prior art.
In contrast to the image formation of the electrophotographic type or the like in the prior art, it is not necessary to melt the toner for fixing it onto a sheet of paper or the like, and a majority of the image forming energy, which is required in such image formation in the prior art, can be saved.
Owing to the above features, the medium can satisfy a current demand for reduction in environmental loads.
The developer contained in the cells includes at least two kinds of developing particles having different optical reflective densities. Further, the developing particles are dry particles, and one kind of the developing particles can appropriately screen or hide the other kind of developing particles. Therefore, image display in good contrast can be achieved.
The developer contained in the cell includes at least two kinds of chargeable dry developing particles, which can be frictionally charged to have different chargeable polarities. For image display, the developing particles which are mutually reversely charged by the frictional charging are easily moved by a Coulomb force. This also achieves image display in good contrast, can suppress remaining of the last image, also allows quick image display, and further can reduce a drive voltage required for image display.
The dry developing particles can suppress settling and condensation as compared with, e.g., electrophoretic particles in a display liquid used for electrophoretic image display, because the liquid is not present. This also suppress lowering of the contrast of the image display, and thereby can perform stable image display for a long time. Since the settling and condensation of the developing particles are suppressed, the remaining of the last image can be suppressed. As compared with the particles in the liquid, the dry developing particles enables stable image display for a long time also for the reason that the charging performance thereof changes with time to a smaller extent.
As compared with the image display by a conventional CRT display or the like, easy-on-the-eyes image display in high resolution can be performed.
The developer-containing type reversible image display medium can be prepared, for example, by a method comprising the steps of forming a substrate having concavities on one surface for forming developer-accommodating cells, accommodating a predetermined amount of the dry developer in the concavities and fixing another substrate to the substrate to form cells accommodating the dry developer.
Another method comprises the steps of forming a substrate having, on one surface, concavities for forming developer-accommodating cells, fixing another substrate to the substrate to form developer-accommodating cells between the two substrates, accommodating a predetermined amount of the dry developer in the cells and closing the cells.
However, these methods have the following difficulties (a), (b) and (c).    (a) It is desirable that the reversible image display medium is thin in its entirety because it is easy to handle, and the partition wall for forming developer-accommodating cell is thin and high (a high aspect ratio) to retain more developing particles for good image display. Accordingly, it is desirable to form a substrate having, on one surface, concavities for forming developer-accommodating cells so that the substrate body is as thin as possible and the partition wall is high (a high aspect ratio). However, actually it is very difficult to form such substrate.
To obtain a substrate having projected and dented portions including concavities on one surface of the substrate, such substrate may be made of a resin material and may be given an uneven surface by transfer molding operation.
However, if the substrate body is made in such a manner that it is thin and the partition wall is high (a high aspect ratio), the following problems arise.    (a-1) In molding operation, the substrate is heated in its entirety up to a melting point of the resin material, so that the substrate warps and the molding die takes a long time for cooling after molding operation.    (a-2) The substrate is liable to become broken or damaged in mold release operation.    (a-3) Because of construction at a high aspect ratio, a substrate having a stable shape is not easily formed.    (b) It is desirable to finely make the concavities for forming developer-accommodating cells on one surface of the substrate in the former method, and it is desirable to finely make the cells in the latter method in order to prevent the undesired movement of the developer on the substrate surface. However, it is very difficult to precisely and smoothly accommodate the predetermined amount of the developer into the fine concavities or cells. In short, difficulties would be encountered in producing a reversible image display medium which allows the precise accommodation of predetermined amount of developer in the developer-accommodating cells.    (c) The following operation may be usually attempted in the former method. A substrate having concavities for forming developer-accommodating cells on its one surface may be formed, the predetermined amount of dry developer may be accommodated in the concavities, another substrate coated with an adhesive material may be superimposed on the substrate, and the substrates are fixed together with the adhesive material, or another substrate coated with an adhesive may be superimposed on the substrate and they may be fixed together with the adhesive before the adhesive cures. In these cases, numerous developing particles adhere to the adhesive material used for fixing the substrates or to the uncured adhesive so that the adhered developing particles are not moved any longer and it becomes difficult for other developing particles to move due to the unmoved developing particles. Further the contrast in image display is lowered and the quality of image is impaired.